Communication receivers are known to comprise an antenna and a signal receiver that includes well-known receiver front-end and back-end circuitry, such as filters, frequency down-converters, analog-to-digital converters, and signal processing circuitry (e.g., a microprocessor or a digital signal processor). In a mobile application, the antenna typically comprises a one-quarter wavelength monopole that is characterized by a corresponding antenna pattern. However, at a base site, the antenna might alternately comprise an array of antenna elements, such as one-half wavelength dipoles, that are collectively characterized by an overall antenna pattern. The antenna pattern of each respective antenna determines the direction, or directions, from which a signal might be received by each antenna. For example, in the case of a one-quarter wavelength monopole projecting upward from an automobile's roof, the antenna pattern provides substantially omnidirectional reception in the horizontal plane.
As is also known, communication receivers are used to receive radio frequency (RF) signals that have been transmitted through an RF transmission medium, such as air. However, due to the presence of reflective objects between the transmitter and the communication receiver, the received signal is often altered in magnitude and phase as compared with the corresponding transmitted signal. This alteration--commonly referred to as fading--results from multiple reflections of the RF signal during transmission through the RF transmission medium. These reflections typically result from obstacles in the signal's path, such as walls, automobiles, or buildings, and may produce multiple modified replications of the signal, each introducing various amplitude and phase alterations of the original signal in each new signal path. All of the signal replicas form a composite signal at the communication receiver's antenna and account for the fading.
In order to mitigate the effects of fading, radio communication receivers typically employ diversity techniques to enhance the signal-to-noise ratio of the signal in a fading environment. Standard diversity techniques attempt to obtain multiple, decorrelated replicas of the transmitted signal--for example, by using multiple antennas typically spaced several wavelengths apart or by receiving redundant transmissions at predetermined time intervals. Thus, by receiving multiple copies of the transmitted signal, the diversity receiver produces an output signal with a better overall signal-to-noise ratio than if only one copy of the transmitted signal were received.
Three common diversity techniques are space diversity, time diversity, and frequency diversity. Space diversity is a technique typically used in a communication unit's (e.g., a mobile radio's) receiver, wherein at least two receiving antennas are separated in space by at least one-quarter wavelength and are used to receive decorrelated replicas of a transmitted signal. The signal received by each antenna is analyzed by the receiver to determine which of the received signals is more preferable. This analysis typically encompasses measuring a received signal strength indication (RSSI) of each signal. Subsequent to the analysis, each signal is weighted based on its respective RSSI, with a higher weighting given to the more preferable signal (i.e., the signal with the larger RSSI). The weighting may include attenuating, or even eliminating, one of the received signals. The weighted signals are then combined to provide a composite signal with an improved overall signal-to-noise ratio. Thus, space diversity requires electrical and mechanical hardware for at least two antennas. Since this additional hardware can be quite costly, users of communication receivers are often deterred from incorporating the improved reception performance provided by space diversity into their receivers.
Time diversity may be employed by a receiver in either a communication unit or a base station. Time diversity uses only one antenna, but requires the same information to be transmitted two, or more, times. The receiver independently receives each transmission and determines the respective signal quality of each received signal, typically via an RSSI. Similar to space diversity, each signal is subsequently weighted based on its respective RSSI and combined to provide a composite signal with an improved overall signal-to-noise ratio. However, due to the redundant transmission requirements of time diversity, the quantity of information (i.e., throughput) that may be transmitted during a particular time period is significantly limited by the necessity to transmit the same information multiple times.
Frequency diversity may also be used by a receiver in either a communication unit or a base station. Similar to time diversity, frequency diversity requires only one antenna. However, frequency diversity also necessitates the availability of multiple frequencies over which to transmit a signal. With frequency diversity, a signal is simultaneously transmitted by a transmitter over multiple frequency channels, wherein each frequency channel is sufficiently separated (typically by at least 1 MHz) in frequency to achieve decorrelation of the transmitted signals. The receiver simultaneously receives each transmission and determines the respective signal quality of each received signal, typically via an RSSI. Similar to space diversity and time diversity, each signal is subsequently weighted and combined to provide a composite signal with an improved overall signal-to-noise ratio. However, since frequency diversity requires the allocation of multiple frequencies, frequency diversity is inherently spectrally inefficient. That is, fewer communications can be transmitted in a particular frequency bandwidth, effectively resulting in higher costs to transmit in that bandwidth.
Therefore, a need exists for a method and apparatus that provides diversity gain to a received signal while remaining spectrally efficient, while facilitating high throughput capability, and while requiting only one antenna.